Particualte SiO.sub.2, whether it be natural or manmade crystalline (quartz) or amorphous (fused quartz or fused silica), typically contains external impurities, internal impurities (occluded and interstitial) and lattice impurities. High purity particulate SiO.sub.2, containing low or undetectable levels of these types of impurities, is required in substantial quantities for the manufacture of fused quartz and fused silica, which materials, in turn, are used in the manufacture of semiconductor grade silicon materials and components, optical fibers for communication, lighting products, optical components and laboratory ware.
In conventional practice particulate silicon dioxide, whether manmade or naturally occuring, crystalline or amorphous, is purified by an acid leach in open, not pressurized leach tanks, using hydrofluoric acid alone or mixed with an accompanying mineral acid. After several hours of leaching the residual acids and fluorides are diluted by decanting at least five times, separated from the silicon dioxide and treated in a waste disposal system. The particulate SiO.sub.2 is then dried and used as is.
The principal drawback of this conventional process is the limitation with regard to the temperature at which the leach can be carried out without excessive losses of acids through evaporation into the environment. Not only are the escaping vapors harmful to human, animal and vegetable life, they are also harmful to the plant environment in which this operation is carried out. The loss of acid vapors is also detrimental from the view point of economics in that it necessitates additions of make up amounts of acid. If carried out at higher altitudes, the evaporation of acids occur at even lower temperatures than at sea level and the problem is thereby exacerbated.
The low temperature limitations of the conventional, open system leach process result not only in longer reaction times, but also prevent adequate attack and suitable reduction and/or elimination of certain common impurities within the batch due to their low solubility at lower temperatures.
In another prior art process, perceived and described as the state of the art continuous process, a stream of particulate silicon dioxide and full strength or dilute hydrofluoric acid and/or acid mixtures are contacted in a counter flow arrangement in a closed, but not pressurized system. Due to the relatively short exposure of a specific amount of silicon dioxide to a specific amount of acid, it is necessary to recirculate the acid over and over again over the silicon dioxide in order to completely use it up, while new acid is continuously added to maintain a suitable rate of attack.
The inventor here has discovered a major drawback of the prior art "continuous" process, which involves mixing fresh with partially spent acid or acid mixtures, in that the increasing amount of dissolved impurities has a strong tendency to inhibit the attack of even the newly added acid which then results in inferior purification as compared to a batch process. In the prior art "continuous" process the originally rather pure reagents become increasingly contaminated with dissolved impurities and larger amounts of wash water are required for the removal of the added contaminants than in the older batch process. The prior art "continuous" process is furthermore limited to lower temperatures for it is not feasible to maintain counter flows at high temperatures and pressures as well.